1. Field of the Invention
The invention relates to implantable prostheses. More particularly, the invention relates to an implantable pelvic prosthesis including an acetabular component, as well as tools and methods for implanting the prosthesis. The prosthesis is used for reconstruction of the hip and hip joint after resection of the pelvis. It provides a stable link between the femur and the pelvis without resorting to an excision arthroplasty.
2. Brief Description of the Prior Art
The hip joint is a ball-and-socket type joint in which the ball-shaped femoral head is engaged with and articulates with a cup-shaped socket known as the acetabulum. Injury and/or disease may damage the hip joint (and/or indeed the pelvis itself) to the extent that the joint, portions of the pelvis and/or combinations thereof must be replaced by or be augmented with a prosthetic device.
Furthermore, deterioration of the acetabulum itself, and particularly the cartilage within the acetabulum, requires that a prosthetic acetabular shell be mounted within a prepared area of the acetabulum. The acetabular shell receives and articulates with a prosthetic femoral head which is installed on a proximal portion of a patient's femur.
In some instances, degenerative bone conditions deteriorate the acetabulum, and particularly its medial wall, to the extent that the acetabulum does not have the integrity to serve as a mounting platform for a prosthetic acetabular shell. Thus, the acetabular component of the prosthesis must be designed to securely attach to whatever bone mass is available. Such mass may be severely limited in instances, for example, where the pelvis is ravaged by cancer. There have been many different proposed designs for an acetabular component and pelvic prostheses suitable for use in the above described situations. The following examples serve to illustrate the state of the art.
U.S. Pat. No. 4,245,360 discloses a partial pelvic prosthesis having an implant piece corresponding to that part of the pelvis to be resected, the piece having a receiving space for the acetabular fossa and connecting bores, and a recess for connecting prosthetic elements whereby the elements can produce the required connection between the implant piece and the pelvis.
The invention disclosed in the '360 patent has threaded bores to receive screws for adjustably fixing prosthetic elements in bores and recess. It is a disadvantage of this device that it chews up bone stock. This actually exacerbates the condition sought to be overcome and greatly complicates revision surgery (replacement of a failed or worn prosthetic component).
U.S. Pat. No. 4,645,507 discloses a prosthesis which has a shaft which carries a bearing at one end. The bearing has a rim at its junction with the shaft. The end of the bearing opposite the rim has a saddle-shaped portion including a pair of protrusions which flank a depression. The depression has a pair of bearing surfaces separated by a protuberance which is formed in the depression and extends from one of the protrusions to the other.
According to the teachings of the '507 patent, the prosthesis is installed at a hip joint by inserting the shaft into the end of the femur nearest the pelvis. The shaft is pushed into the femur until the rim on the bearing contacts the end of the femur. One of the protrusions is passed through an opening in the wall of the pelvis so that the wall is received in the depression of the bearing.
The wall is contoured so as to conform to the shape of the depression and the protuberance therein and rests on the surface of the protuberance as well as the bearing surfaces of the depression. When the prosthesis has been installed in this manner, relative pivotal movement of the femur and the pelvis is possible.
The prosthesis described in the '507 patent makes it possible to obtain relative pivotal movement of the femur and the pelvis even when the pelvis has been damaged or has deteriorated to such an extent that the natural socket can neither be repaired nor replaced. However, the implant is unstable and easily dislocates.
U.S. Pat. No. 5,030,238 discloses a hip prosthesis wherein a saddle-shaped head has a seat flanked by two horns and engageable with the surface bounding a recess in the lower part of a damaged pelvic bone. The head is rotatably or rigidly secured to a substantially S-shaped adapter which, in turn, is rotatably or non-rotatably secured to the adjacent end of a shank that is implantable in the cavity of a femur. One or more distancing rings can be inserted between the adapter and the head.
The device described in the '238 patent was intended to overcome the shortcomings of the device described in the '507 patent.
In particular, as stated in the '238 patent:
“ . . . heretofore known prostheses with saddle-like heads fail to satisfy all of the requirements which must be fulfilled by an artificial hip joint.
First of all, when the wearer of the prosthesis is walking, relative movement between the head of the prosthesis and the socket of the pelvis entails a pronounced mechanical stressing of the remainder of the pelvis. When the pelvis and the femur perform large movements relative to each other, the horns of the saddle-like head of the implanted prosthesis strike the adjacent portions of the pelvis.
Secondly, frictional engagement between the head of the implanted prosthesis and the adjacent portion of the pelvis brings about extensive wear upon the pelvis; in fact, the head is likely to penetrate into the pelvis and to shorten the respective lower extremity of the patient.”
Furthermore, according to the 238' patent:
“It was further discovered that, when the pelvis has undergone extensive damage (either as a result of an accident or as a result of illness), the muscles in the region of an implanted conventional prosthesis with a saddle-like head can exert only relatively small forces in a sense to straighten out the extremity into which the prosthesis is implanted because they are incapable of finding an appropriate lever arm for the application of conversion or transmission forces which are being generated thereby. Therefore, a patient wearing such a prosthesis is likely to limp because she or he must continuously strive to maintain the center of gravity of the body above the vertical axis which is common to the saddle-like head and the shank of the implanted artificial hip joint.”
Despite the recognition of many of the disadvantages of the prior saddle-like prostheses, the solution proposed by the '238 patent is not ideal. It requires the use of many pins. This results in bone loss which complicates revision surgery. In addition, the prosthesis of the '238 patent provides only limited movement.
U.S. Pat. No. 5,326,367 discloses an endoprosthesis for cancer damaged hip bones which has a recess in a distal portion to receive a prosthetic hip socket.
The endoprosthesis described in the '367 patent includes an individual distal part and an individual proximal part which are secured to each other by means of a screw connection. The distal and the proximal part include mounting brackets for screwing the parts to the hip bone or, respectively, to a vertebra. In addition, the proximal and distal parts are shaped to positively inter-engage each other. This device overcomes the problems inherent in the saddle-like prostheses but requires the use of a number of screws to affix it to the hip bone.
U.S. Pat. No. 5,871,548 discloses a modular acetabular reinforcement system having a substantially cup-shaped reinforcement body with a peripheral flange portion. One or more fixation wings, of various sizes and shapes, are selectively and separately attachable to the flange portion of the reinforcement body.
The system disclosed in the '548 patent is mountable within the acetabulum of a patient to reinforce the acetabulum and to serve as a platform for other prosthesis components such as an acetabular shell. This device is similar to the device described in the '367 patent insofar as it is not a saddle-like prosthesis and it is affixed to the bone with many screws. It is an advantage that the “wings” may be affixed to extend radially from almost any location on the peripheral flange. The disadvantage is that it requires the use of a number of screws to affix it to the hip bone.